Data stored on an optical disc can be read out from the disc by irradiating the rotating disc with a relatively weak light beam with a constant intensity, and detecting the light that has been modulated by, and reflected from, the optical disc.
On a read-only optical disc, information is already stored as pits that are arranged spirally during the manufacturing process of the optical disc. On the other hand, on a rewritable optical disc and a write-once optical disc, a recording material film, from/on which data can be read and written optically, is deposited by an evaporation process, for example, on the surface of a base material on which tracks with spiral lands or grooves are arranged.
In writing data on such a rewritable or write-once optical disc, data is written there by irradiating the optical disc with a light beam, of which the optical power has been changed according to the data to be written, and locally changing the properties of the recording material film.
It should be noted that the depth of the tracks and the thickness of the recording material film are smaller than the thickness of the optical disc base material. For that reason, those portions of the optical disc, where data is stored, define a two-dimensional plane, which is sometimes called an “information storage plane”. However, considering that such an “information storage plane” actually has a physical dimension in the depth direction, too, the term “information storage plane” will be replaced herein by another term “information storage layer”. Every optical disc has at least one such information storage layer. Optionally, a single information storage layer may actually include a plurality of layers such as a phase-change material layer and a reflective layer.
To write data on a rewritable optical disc or a write-once optical disc or to read data that is stored on such an optical disc, the light beam always needs to maintain a predetermined converging state on a target track on an information storage layer. For that purpose, a “focus control” and a “tracking control” are required. The “focus control” means controlling the position of an objective lens perpendicularly to the information storage layer such that the focus position of the light beam is always located on the information storage layer. On the other hand, the “tracking control” means controlling the position of the objective lens along the radius of a given optical disc (which direction will be referred to herein as a “disc radial direction”) such that the light beam spot is always located right on a target track.
Various types of optical discs such as DVD (digital versatile disc)-ROM, DVD-RAM, DVD-RW, DVD-R, DVD+RW and DVD+R have become more and more popular these days as storage media on which a huge amount of information can be stored at a high density. Meanwhile, CDs (compact discs) are still popular now. Recently, there are increasing demands for optical discs with storage capacity that is big enough to store high-definition data, and Blu-ray Disc (BD) has been developed to meet such demands. Although BD-RE, which is a rewritable optical disc, has already been put on the market, BD-R, which is a write-once optical disc that can be produced at a lower cost than BD-RE, has also been under development.
In write data on an optical disc with a narrow track pitch such as a BD, the tracking control on such an optical disc needs to be carried out with particularly high precision. The tracking control might lose its stability due to the impact the optical disc is subjected to while data is being written there, defects on the optical disc or some scratches or dust on the surface of the disc. If the tracking control lost its stability for any of these reasons while data is being written on a target track, then the converged spot of the light beam might go off the track and the data could be written on an adjacent track by mistake.
In the prior art, if the magnitude of the tracking error that has been detected based on the magnitude of a tracking error signal is equal to or greater than a predetermined value, it is determined that “track skipping” have occurred and that the data stop being written immediately.
According to the technique disclosed in Patent Document No. 1, if defect detecting means has detected defects on an optical disc, even an increase in the level of a tracking error signal is regarded as resulting from those defects, not due to “track skipping”. On the other hand, Patent Document No. 2 discloses a technique for sensing “track skipping” by detecting address pits that are arranged between tracks.                Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 2003-272161        Patent Document No. 2: Japanese Patent Application Laid-Open Publication No. 2001-56936        